I think this is highly individual to the child-family system. My extended family, including in-laws, contains quite a number of persons who either attended college early, or could have, but did not, for various reasons. The differences between the successful early college students and others are not always in the cognitive/conceptual realm, as you note. Areas of academic strength, emotional maturity, temperament, executive functions, access to effective scaffolding, peer social group, pace/nature of college entry (e.g., gradual vs complete matriculation, online/distance learning vs in-person, small CC classes vs 800 person lectures) are all factors that appear to have affected outcomes, or at least the path there.
My own experience was padded by a built-in peer group (sibs and other relatives who attended college at the same time, some of whom were early entrants at the same institution), and sufficient EF scaffolding by my parents (their safety net had just enough holes in it for me to experience some much-needed natural consequences, without allowing catastrophic failure). I also had the benefit of one audit course about three or four years earlier, and one summer course a year earlier, prior to full college entry, but still had to make a lot of adjustments. I think the earlier trial courses gave my parents some hard data on my progress toward preparation for college entry.
Though my experience was net positive, I am not inclined to have our children step into early college for the first time as fully matriculated freshmen. We're likely dipping our toes into college coursework this coming year as well, one class at a time, in interest areas, most likely STEM, starting from the most resilient and socially-mature child.
Perhaps AOPS would be more rigorous through calculus, but once learners pass calc, options outside of enrolling in college coursework become much more limited (MIT OCW being a notable exception--but that's essentially college coursework, without the in-person instruction, credit, or tuition fees). Also, one begins to enter territory where documenting college credit may have particular value.
In my FOG experience, those whose personal strengths were in STEM fields just put off all of their humanities requirements until the last year, loading up on extra STEM coursework early on. Even if a learner starts this process as a preteen (as some in my family did), natural development occurring with the passage of time mostly resolves the question of emotional maturity as it applies to humanities coursework by senior year. (It does not, of course, resolve the question of adult themes in interpersonal interactions with peers beginning in year one. Some families have addressed this by having one parent devote extensive time to accompanying the child to class. In my FOG, we had, as noted, co-enrolled family to support and shelter each other.)
